A voltage controlled oscillator (VCO) or oscillator is a component that can be used to translate DC voltage into a radio frequency (RF) voltage or signal. The magnitude of the output signal is dependent on the design of the VCO circuit and the frequency of operation is determined, in part, by a resonator that provides an input signal. In general, VCOs are designed to produce an oscillating signal at a particular frequency ‘f’ that corresponds to a given tuning voltage. In particular, the frequency of the oscillating signal is dependent upon the magnitude of a tuning voltage Vtune applied to a tuning diode network across a resonator circuit. The frequency ‘f’ may be varied from fmin to fmax and these limits are referred as the tuning range or bandwidth of the VCO. The tuning sensitivity of the VCO is defined as the change in frequency over the tuning voltage and it is desirable to tune the VCO over a wide frequency range within a small tuning voltage range.
Clock generation and clock recovery circuits typically use VCOs within a phase locked loop (PLL) to either generate a clock from an external reference or from an incoming data stream. VCOs affect the performance of PLLs. In addition, PLLs are typically considered essential components in communication networking as the generated clock signal is typically used to either transmit or recover the underlying service information so that the information can be used for its intended purpose. PLLs are also important in wireless networks as they enable the communications equipment to quickly lock onto the carrier frequency on which communications are transmitted.
The popularity of mobile telephones has renewed interest in and generated more attention in wireless architectures. This popularity has further spawned renewed interest in the design of low noise wideband oscillators. In that regard, most mobile communication systems include a tunable VCO as a component in a frequency synthesizer, which selectively provides a choice of the desired channel. The recent explosive growth in the new families of cellular telephones and base stations using universal mobile telephone systems (UMTS) has stirred a need for developing an ultra-low noise oscillator with a fairly wide tuning range. The demands of wideband sources have generally increased telescopically because of the explosive growth of wireless communications. In particular, modern communication systems are typically multi-band and multi-mode, therefore requiring a wideband low noise source that preferably allows simultaneous access to DCS 1800, PCS 1900 and WCDMA (wideband code division multiple access) networks. The commercial handsets employed by these and other next generation networks are typically required to be capable of handling not only voice data, but also image and video data. Therefore, the radio link typically has to deal with signals that are more digitally complex.
The dynamic operating range and noise performance of a VCO may limit or affect the performance of the PLL itself, which in turn may affect the performance of the device in which the PLL is employed, e.g., RF transceivers, a cell phone, a modem card, etc. Broadband tunability of VCOs represents one of the more fundamental tradeoffs in the design of a VCO, impacting both the technology and the topology used. The dynamic time average quality factor (i.e., Q-factor) of the resonator as well as the tuning diode noise contribution affect the noise performance of a VCO. Furthermore, the dynamic loaded Q is, in general, inversely proportional to the operating frequency range of the VCO.
Despite the continuous improvement in VCO technology, low phase noise typically remains a bottleneck and poses a challenge to RF transceiver (transmitter—receivers) design. In addition, oscillator/VCO design typically poses a challenge to the RF trans-receiver system. This is typically considered due to the more demanding parameters of the VCO design: low phase noise, low power consumption and wide frequency tuning range. For example, in a receiver, the phase noise of the local oscillator limits the ability to detect a weak signal in the presence of a strong signal in an adjacent channel. In a transmitter, phase noise results in energy being transmitted outside the desired channel or band.
Improvements in oscillator/VCO technology have continued with time, yielding ever-smaller sources with enhanced phase noise and tuning linearity but the phenomena of the thermal drift over the temperature range (−40° C. to +85° C.) has not been properly addressed. The wide operating temperature range of the oscillator/VCOs coupled with a general lack of information on the thermal drift-profile creates a need for the development of a uniform and user-definable thermal drift profile oscillator with a relatively low thermal drift over the wide operating temperature range and operating frequency band.
Usually, high-stability oscillators are built with a quartz crystal up to frequencies of several hundred megahertz. However, in order to achieve better overall performance, the SAW (surface acoustic wave) resonator based oscillator is generally considered a better choice for an ultra low phase noise low thermal drift oscillator. In that regard, SAW resonator based oscillators are widely used in wireless applications, since that technology generally features relatively low phase noise at fixed frequencies, low microphone noise (tolerance to vibration), high Q and low jitter.
On the other hand, SAW resonators are typically used in oscillators as a two-port resonator and have a relatively small pull-in range that usually does not support a wide enough tuning range to compensate for tolerances due to the circuit components. This typically limits the amount of correction that can be made to compensate for the tolerances of the circuit components and thermal drift over the operating temperature range (−40° C. to +85° C.). In addition, SAW devices are comparatively expensive compared to CROs (ceramic resonator based oscillator) and their availability and performance are limited to a select frequency and a relatively narrow operating temperature range (−20° C. to +70° C.). This generally makes them unsuitable for operating in stringent temperature environments and/or low cost applications.
Thus, there is a need for a low noise, low thermal drift oscillator that is operable over a wide temperature range and which offers a cost-effective solution to the demand for a low noise tunable oscillator.